An organic electroluminesence display element (hereinbelow referred to as organic EL display element) is an extremely thin, self-light-emitting element. Compared with liquid crystal display elements which are spreading as thin, full-color display elements widely, this organic EL display element is superior in a view angle and speed of response. In this way, it is possible to display at high level, so that the luminous display element such as the organic EL display element is being developed actively so as to be applied to a high-quality display.
The organic EL display element is arranged by providing an organic electroluminesence layer (hereinbelow referred to as organic EL layer) between a transparent electrode provided on the front side and a metal electrode provided on the back side. The organic EL layer is made up of plural thin films such as a hole injection layer, a hole transport layer, an emission layer, an electron transport layer, and an electron injection layer. These plural thin films are formed sequentially by vacuum evaporation method or sputtering method, and deposited on a transparent substrate of the front side on which the transparent electrode is provided. Further, a method by which the thin films are formed by printing at low cost is proposed recently.
In this way, the organic EL display element is generally made up of at least one organic EL layer and a pair of electrodes, and an electron is injected from an electrode and a hole is injected from another electrode. A recombination of the injected electron and the injected hole causes the emission layer to emit light.
Although the organic EL display element is a promising art as a flat panel display, at the same time, it has problems associated with luminance of a material which makes up the organic EL layer, improvement of emitting efficiency, and life-lengthening etc. A conventional organic EL display element, for example, employs the following structure.
A luminous polymer display is described in “Society for Information Display '99 Digest” (pp. 372-375) (prior art (1), published in 1999). In this document, a flat display which has high resolution,’ a wide view angle, high contrast ratio, and high speed of response is realized by combining a thin transistor made of polycrystal silicon and a luminous polymer.
However, when an incident angle of light which is incident on a substrate on the side of an observer from which the light emerges, and on an electrode through which the light passes is over a critical angle, the light which is emitted from the organic EL layer such as the luminous polymer is totally reflected, so that the light emitted in the organic EL display element emerges less efficiently. Thus, in order that the light emerges more efficiently, the organic EL display element employs the following structure.
Japanese Patent No. 2692671 (prior art (2), publication date: Sep. 5, 1997) discloses a resonator type organic thin film EL element. In the publication, it is described that the light which is emitted from the organic EL layer emerges more efficiently by providing a resonator such as a multilayer film reflecting mirror.
Further, Japanese Unexamined Patent Publication 2773720 (prior art (3), publication date: Apr. 24, 1998) discloses an organic thin film EL element. In the publication, it is described that the emitted light emerges more efficiently by providing a lens on the light-emerging side of a substrate.
Further, Japanese Unexamined Patent Publication 2991183 (prior art (4), publication date: Oct. 15, 1999) discloses an organic EL display element in which a diffraction grating and a zone plate are provided. In the publication, it is described that the emitted light emerges more efficiently by providing the diffraction grating and the zone plate.
Further, “Society for Information Display '99 Digest” (pp. 185-187)(prior art (5)) discloses an organic electroluminesence device in which three organic EL layers are deposited. In this document, it is described that high resolution can be realized by depositing the organic EL layers in three-phase. However, it is required to deposit a transparent organic EL layer as the second layer from the front side in order to deposit three organic EL layers. Thus, when the electroluminesence device does not emit light, 60% to 80% of transmittance of light is realized by depositing extremely thin metal electrodes. Although the extremely thin metal electrode brings about a high resistance value, an ITO is deposited on the metal electrode so that the resistance value is low.
Further, ambient light which is incident on the organic EL display element is reflected in the electrode. When the reflected light is reflected in a direction of an observer, an image is reflected on the organic EL display element. In order to prevent the reflection of an image, the organic EL display element employs the following structure.
In Japan Unexamined Patent Publication 2000-40584 (prior art (6), publication date: Feb. 8, 2000), there is a description of an organic EL display element in which it is possible to prevent reflection of an image due to mirror reflection of the metal electrode by providing irregularities on the transparent electrode, the metal electrode, and the organic EL layer.
Further, in Japan Unexamined Patent Publication No. 321381/1996 (Tokukaihei 8-321381) (prior art (7), publication date: Dec. 3, 1996), there is a description of an organic electroluminescence which can absorb the ambient light which passes through the organic EL layer and is reflected by the metal electrode by providing a polarizing film on a front surface of the organic electroluminescence. Thus, it is possible to prevent reflection of an image due to mirror reflection of the metal electrode, and prevent degradation of contrast ratio.
Further, Japan Unexamined Patent Publication 205322/1995 (Tokukaihei 7-205322)(prior art (8), publication date: Aug. 8, 1995) discloses an electroluminescence display element. In a structure described in the publication, a function which reflects the incident light in an incident direction (retro-reflection) is provided by providing a microcorner cube array which is made of minute protrusions in a form of a triangular pyramid. Thus, even when the ambient light is incident, it is reflected in a direction of a light source, so that the ambient light does not disturb observation of an observer. Thus, reflection of an image in the electroluminescence display element is prevented.
However, generally, a thickness of an EL layer is not more than 200 nm. In this way, the EL layer is extremely thin, so that most of the ambient light passes through the organic EL layer of the prior arts (1), (2), (3), (4), and (5). The metal electrode is flat, so that a surface of the metal electrode is like a mirror. Therefore, the ambient light having passed through the organic EL layer is reflected by the metal electrode and is emitted to outside of the display element. Thus, like a mirror, reflection of an image occurs in the organic EL display element, so that a visibility of display is much spoiled. Further, when the strength of the ambient light such as outdoor light is larger than the strength of the light in the organic EL layer, contrast ratio much degrades.
In the prior art (6), irregularities in a form of dots are provided on the transparent substrate at random, so that it is possible to suppress a degradation of a visual quality due to the reflection of an image which is brought about by the ambient light. However, ambient light is reflected by the metal electrode also in a direction of an observer, so that contrast ratio of display degrades.
Further, techniques for preventing the reflection of an image brought about by the ambient light and the degradation of the contrast ratio includes a technique by which reflectance ratio of the metal electrode is dropped, and a technique by which, as disclosed in the prior art (7), a polarizing film and a quarter wavelength retardation film are provided on a front surface of the organic electroluminescence so that the polarizing film absorbs the reflected the ambient light. However, by these techniques, the metal electrode and the polarizing film absorb the light which is emitted by the organic EL layer, so that utilization efficiency of emission declines.
In a structure of the prior art (8), the display device is provided with a corner cube array whose unit structure is a form of a triangular pyramid. The corner cube array is not packed in the closest manner, so that it is required to keep the ambient light away from a portion which has no corner cube array. Even if the closest packing is performed, when the corner cube array whose unit structure is a form of a triangular pyramid is used as it is, there exists a portion which has no retro-reflectivity (non-retro-reflective portion). The ambient light which is reflected by this portion spoils a black state of a display device, so that contrast ratio degrades.
Further, the corner cube array is provided on the front side with respect to the EL layer. By this, a portion which has retro-reflectivity (retro-reflective portion) shields the light which is emitted in the back side, so that the light is shielded in two-thirds of an area of the corner cube array, so that utilization efficiency of emission degrades. Further, when a light shielding process etc. is performed on the non-retro-reflective portion of the corner cube array so that the ambient light is not reflected by the non-retro-reflective portion, the light emitted from the EL layer cannot emerge toward the side of an observer.
Thus, by the techniques of the prior arts, it is impossible to obtain a luminous display element which does not allow an image to be reflected, and has high contrast ratio of display and high utilization efficiency of emission.